I too have had some problems in this "area". Something I have found that works nicely is to stay AWAY from cotton under garments. Cotton holds moisture, and moisture is what causes the rash. Polyester, Mesh-type, breathable briefs have all but eliminated this issue for me.

Get some good fitting briefs or fitted boxer-briefs made out of synthetic material (the men folk can correct me if I'm wrong on this fit suggestion). There are also some wool and wool blend underwear, but I haven't tried those for personal underwear yet and if your skin is sensitive, it might not be the first thing you should try.

Anyway, there are a lot of non-cotton options out there. I really like the women's capilene hipsters from Patagonia, but you can get others from EMS and other stores. They may cost more, but buying a pair or two is definitely worth it and if they're decent quality, they'll last a long time. Look for sales and overstocks and buy whatever color/print they have in your size, if you want to save money.

After you address the underwear issue, make sure your outer pants/shorts layer is also non-cotton and isn't causing further chafing or moisture retention.

+1 for ExOfficio briefs. Very breathable, good fit for me, and one can literally wash them with a little water and Dr. Broners, wring, and wear again. My body heat dries them in very short time, with no adverse effects.

+1 for a lubricant. There can be some animosity at first, but the alternative option is much worse when you really think about it; lube up.

Umm...continuing the lubricant mentions, I use BodyGlide sometimes on my inner thighs for running. It definitely reduces or eliminates any irritation from the occasional chafing.

I'd deal with the non-cotton underwear issue first though, since that's likely most of the original poster's problem.

Oh, and one more suggestion from a guy friend of ours who's done some adventure racing: lightweight synthetic compression shorts alone under your pants/shorts (not bike shorts though). I'm not a guy and I've never done it myself though, so take that as you like it.

Oh, and whatever you try, watch out for bad seams. You want seamless construction and paying a little more for decent fitting and designed underwear can be well worth it.

I wear briefs made by Jockey. Not sure what model-not the skimpy ones made of cotton though; these are made of a synthetic blend and are great for winter camping or skiing. They have about a one inch waistband and say Jockey across the front in block letters. Very comfortable and no problems with heat rash from sweating.

Well I ware boxer briefs they don't ride up or chafe. You could also try regular boxers. You also look into skivies mad of a wicking material this also may help. Because the mostly cotton material holds the moisture from your sweating and the wicking material would help move it away from you body. But for me when its hot and humid I usually go commando, Its cooler and I get more air circulation. Because of the fact that I am a heavy sweater.

Regular boxers are killer on the trail. They bunch up and entangle your junk and who needs that. I've since bought some Underarmour synthetic boxer briefs with a tight fit. I don't particularly like to wear them (I prefer to free-ball it), but they eliminate chaffing and wick moisture, and that's a godsend while backpacking.

Also, you might just try to go without anything, as trouthunter suggested. I prefer that, but sometimes I can still get chaffing on my inner thigh from whatever pants/shorts I happen to be wearing. In that case the synthetic snug boxer briefs are ideal.

I had a similar problem that didn't last long after I discussed it with a doctor. he prescribed a medicated ointment that used regularly for a few weeks. rash went away and didn't come back until much later. i still had some of the ointment left, so i applied a little and it hasn't been back since. that was a year ago and even during this past summer i didn't seem to have a problem. the ointment isn't terribly expensive if you have insurance and i'd hate to see you change out an entire wardrobe if this would help. good luck to ya and happy hiking!

Keeps me warm even if I'm sweat, dries rapidly, "feels" dry even when wet. Only con is the price, but that hurts only once. I like it so much I use it for everyday-use, even now as I write I have this on.

No cotton is very much the key here.Cotton gets wet and stays that way.As for Gold bond,Vaseline or Butter,the type for bike riding,it is a matter of what you like best.Try the non cotton briefs,many on the market, and play around with the lube to find the best combo for you.

Keep clean. I recently learned about and used the "Combat Bath" antimicrobial towelettes available from Sportsmansguide.com. They work great. One towel can bathe your whole body top to bottom in a waterless environment. Of course, the area in question should be the LAST area you clean :). It's the cleanest I've ever been on a backpack trip (5 nights - minimal funk).

I use Champion synthetic boxer briefs. They are not expensive, light, seamless and conform well to your body to protect you. Or go without.